I'm in a relationship with an ISTP and it's still going strong after 9 months. That's not a lot of time, of course, but we've successfully been through road trips, big holidays, both birthdays, a serious injury-accident, meet-the-mom (his), and (temporary) cohabitation together. So it feels like we've packed in a lot of experience with each other.
It's a bit like whitewater rafting, and every time *I* thought we couldn't make it, we did, and were rewarded with a gorgeous, tranquil vista. I think the INFP tends to do all the worrying about the relationship whenever she encounters the tiniest flaw, whereas the ISTP remains cheerful and optimistic, which is wonderful because it helps stabilize the rocky moments.
Communication is our weak point (and yes, he has to be the extrovert a lot of times), but problems are always quickly solved. We're both patient, flexible and open to letting the other person grow at his own pace, which is essential to each other's happiness. I find that (after a state of trust is reached - might take months) it's a very conflict-free relationship, 99% of the time. What conflict there is, is usually due to my paranoia/insecurity/imagination, or his weak Fe (and he's not beyond acting out on insecurity himself). More importantly, we believe these are things we can work on improving because the relationship is totally worth the effort.
So why is it worth it? I could type a million words on this. There is a lot to the INFP that just works with a mature and patient ISTP. It's better if the INFP is mature too. And truly, if there's anything that's not quite there, it's not a big deal... it's always open to improvement and fine-tuning (this is a central philosophy of ours), and it's entirely of our own free will... no pressure to change. It is more important to just be there for each other. What I often find hard to realize is that just being me is enough for him.
He loves to impress by performance, show off his quick wit, share his knowledge and his world, parade his good looks, and I love to admire, learn, and feel close to him. We both enjoy giving, albeit in different ways. When it comes to emotional stroking or romance, he's terribly efficient (or deficient), but it doesn't take a lot to keep me happy. Just knowing he's got a melty heart deep inside (which peeks out at certain moments) is generally F enough for me. Basically, he knows he's got an amazing girlfriend in me, and I know I've got an amazing boyfriend in him. It's inconceivable right now to imagine we'd give up on something that's so comfy and happy. The future looks long, bright and exciting for us, so long as we both strive toward our personal ideas of awesomeness (which tend to agree) and not let things get stagnant.